1. Field of the invention
The present invention relates to a sheet medium feed controller having a feed error detection function for a sheet medium and an image recording apparatus employing the feed controller.
2. Related Background Art
An elongated recording medium (to be also referred to as a sheet medium hereinafter) wound in a roll is generally used in a recording system of a conventional recording apparatus such as a facsimile apparatus or a printer, and recording is performed with a line record head line by line.
For example, in a recording apparatus shown in FIG. 23, a record sheet 50 supplied from a sheet roll 50a is fed to the sheet feed direction downstream side (to be simply referred to as the downstream side hereinafter) indicated by an arrow a by using a platen roller 51. An ink sheet 53 wound around a supply roll 52 is fed in a direction indicated by an arrow h and is taken up by a take-up roll 54. A record head 55 biased by a spring 55a from its rear surface is urged against the platen roller 51. The record head 55 is energized in accordance with record data to transfer an ink on the ink sheet 53 to the record sheet 50, thereby recording an image. A photo sensor 56 detects the presence/absence of the record sheet 50. A guide plate 57 is arranged opposite to the photo sensor 56. The guide plate 57 is made of a member having a lower reflectance than that of the rear surface of the record sheet 50. A cutter 58 is used to cut a record sheet 50 upon image recording of one page.
An operation of feeding the record sheet 50 in the recording apparatus will be described below. In a record standby state, as shown in FIG. 23, the leading end of the record sheet 50 stands by at an upstream position of the photo sensor 56 but at the downstream side of the platen roller 51. At this time, light emitted from the photo sensor 56 is reflected by the guide plate 57 and is incident again on the photo sensor 56. The level of the reflected light is defined as Low (to be simply referred to as "L" hereinafter).
When recording is started, the platen roller 51 and the take-up roll 54 are rotated to feed the record sheet 50 and the ink sheet 53 in the directions indicated by the arrows a and b, respectively, as shown in FIG. 24. The record head 55 is energized in accordance with record data, and recording is performed on the record sheet 50 in units of lines. When the leading end of the record sheet 50 passes by the photo sensor 56, light emitted from the photo sensor 56 is reflected by the rear surface of the record sheet 50 and is incident on the photo sensor 56. Since the rear surface of the record sheet 50 has a higher reflectance than that of the guide plate 57, the level of this reflected light is defined as High (to be simply referred to as "H" hereinafter).
When the platen roller 51 is rotated by a predetermined amount (i.e., until the leading end of the record sheet 50 is fed from the record standby position to the position of the photo sensor 56), and the input level of the photo sensor 56 is changed from L to H, normal feeding of the record sheet 50 is detected. If the input level is kept at L, a feed error of the record sheet 50 is detected.
When recording of one page is completed, the record sheet 50 is cut by the cutter 58, and the recorded record sheet 50 is ejected outside the apparatus. The leading end of the non-recorded record sheet 50 is fed to the upstream side upon reverse driving of the platen roller 51. The record sheet 50 is rewound to the record standby position shown in FIG. 23.
When recording, cutting, and rewinding are completed, and the input level of the photo sensor 56 is changed from H to L, normal feeding is detected. However, when the input level is kept at H, a feed error of the record sheet 50 is detected.
In the prior art described above, however, detection of a feed state of the record sheet 50 is performed at limited timings. More specifically, this detection is performed when the leading end of the record sheet 50 crosses the photo sensor 56 and when the leading end of the recorded record sheet 50 crosses the photo sensor 56 again. Therefore, the feed state of the record sheet 50 during recording cannot be performed.
For example, even if a trouble occurs in a driving source for the platen roller 51 during recording shown in FIG. 24 and the platen roller 51 is not rotated, the photo sensor 56 cannot detect this trouble. In this case, since the record sheet 50 is stopped, images are repeatedly recorded on the same portion of the sheet, and the recorded information cannot be read. In addition, since heat from the record head 55 is concentrated on one portion, the ink sheet 53 and the record sheet 50 may be damaged.
In order to solve this problem, there is a proposal in which an encoder is arranged at the platen roller 51. As described above, the record sheet 50 is fed in a direction opposite to that of the ink sheet 53. When a friction between the record sheet 50 and the ink sheet 53 becomes larger than that between the record sheet 50 and the platen roller 51, slippage occurs between the record sheet 50 and the platen roller 51 even if the platen roller 51 is normally rotated. As a result, the above problem is posed due to the stop of the record sheet 50. The record sheet 50 may be fed in the same direction as that of the ink sheet 53, i.e., in the direction indicated by the arrow b and may be wound around the take-up roll 54.
The above problem is also posed by a decrease in friction caused by a deterioration of the platen roller 51 over time or a variation in friction between the record sheet 50 and the ink sheet 53 due to changes in temperature and humidity. However, the above trouble cannot be detected by the encoder arranged at the platen roller 51.
The above trouble occurs due to adhesion between the ink sheet and record sheet. More specifically, the heat transfer printer described above uses an ink sheet obtained by coating a thermally meltable (or thermally sublimable) ink on a base film, and the ink sheet is selectively heated by a thermal head in accordance with an image signal. The melted (or sublimed) ink is transferred to record paper to perform image recording. This ink sheet is generally a sheet from which the ink is perfectly transferred in one image recording operation (so-called a one-time ink sheet). When recording of one character or one line is completed, the ink sheet is fed by a length corresponding to the record length, and an unused portion of the ink sheet must be located at the next recording position. For this reason, the amount of the ink sheet used is increased, and the running cost of the heat transfer printer tends to be increased as compared with a normal thermosensitive printer for performing printing on thermosensitive paper.
In order to solve the above problem, as disclosed in Japanese Laid-Open Patent Application Nos. 57-83471 and 58-201686 and Japanese Patent Publication No. 62-58917, there are proposed heat transfer printers in which record paper and an ink sheet are fed at different speeds.
As described in these prior-art inventions, an ink sheet (so-called multi-print ink sheet) capable of performing recording a plurality of times (n times) is known. If this ink sheet is used, when recording is continuously performed in units of record lengths L, the feed length of the ink sheet fed upon completion of recording of each image or during image recording can be set smaller than the length L (L/n for n&gt;1). In this case, the efficiency of use of the ink sheet can be increased by n times the conventional case, and a reduction in running cost of the heat transfer printer can be expected. This record scheme is called a multiprint scheme.
In the multiprint scheme using the above ink sheet, the ink in an ink layer of the ink sheet is heated divisionally n times. Ink transfer is performed by a shear force generated between the melted ink and the nonmelted ink of the ink layer. For this reason, when the ink temperature is decreased due to an increase in time interval between recording of one line and recording of the next line, the shear force between the melted ink and the nonmelted ink is increased, and the ink sheet tends not to be easily released from the record paper. This typically occurs when a large number of black data are included in record data of one line. This poses a problem in a facsimile apparatus or the like in which the time interval between the current line and the next line is not constant and is relatively long. In the worst case, the ink sheet is adhered to record paper. When recording is continued in this state, the ink sheet is damaged, resulting in inconvenience.